Mechanical damping materials remove energy from a system. Motions to be damped can be periodic and regular (e.g., sine wave, square wave) or they can be irregular. Often a single mechanical event must be damped. Such an event is termed a shock, and the mechanical damping is termed shock absorption. Most damping measurements apply a periodic deformation to the article being tested, but it is also possible to assess the damping characteristics of a material from a single shock.
The Young's modulus, E, (also known as elastic modulus, modulus of elasticity, or tensile modulus) is a measure of the stiffness of a material. E is the ratio between the tensile stress, σ, divided by the tensile strain, e. E is typically measured on a tensile apparatus which elongates a material and reports the stress needed to produce a certain strain. Alternatively, a sample is compressed and the required stress for a needed deformation is measured. E may be measured under static, or quasi-static, conditions, where the stress does not vary with time. Alternatively, the modulus can be measured under dynamic or time-varying conditions where a material may exhibit properties of elasticity and viscous flow (viscoelasticity) in which case the modulus depends on frequency of deformation and a complex modulus, E*, is defined, where E*=E1+iE2, where E1 is the storage modulus, which is measure of energy stored on a deformation cycle, and E2 is the loss modulus, which is a measure of the energy lost on a cycle.
There is a need for materials and articles that exhibit damping and/or shock absorbing properties. PCT application WO/2008/027989 discloses the use of materials and articles comprising blends of positive polyelectrolytes and negative polyelectrolytes, also termed “polyelectrolyte complexes.” Polyelectrolyte complexes are prepared in a straightforward manner by mixing solutions of positive and negative polyelectrolytes. The complexes that precipitate from solution have very poor mechanical properties, and PCT application WO/2008/027989 discloses methods for compacting these complexes to provide articles with improved mechanical properties.
A method of preparing thin films of polyelectrolyte complexes was disclosed in U.S. Pat. No. 5,208,111. In this method, a substrate is exposed in an alternating fashion to positive and negative polyelectrolytes. The resulting “polyelectrolyte multilayers” have a composition similar to solution-precipitated polyelectrolyte complexes. However, each layer of polyelectrolyte added to a growing film has an opportunity to complex efficiently and completely with the existing material, excluding additional of water. Thus, the mechanical properties of polyelectrolyte multilayers and solution-precipitated polyelectrolyte complexes differ.
Recent studies have evaluated the mechanical properties of polyelectrolyte multilayers, which are typically less than one micrometer thick. See, for example, Jaber, J. A. and Schlenoff, J. B., J. Am. Chem. Soc. 128, 2940-2947 (2006). The elastic modulus of these films ranges from kPa to MPa. However, these films are far too thin (a few micrometers or less) to be used for mechanical components in most systems.
The maximum amplitude of mechanical damping of an article generally depends on the physical dimensions of the article. Thus, there is a need to prepare articles with dimensions in the millimeter to centimeter scale to absorb the shock of mechanical vibrations on the millimeter scale. While a polyelectrolyte complex is easily prepared by mixing solutions of individual polyelectrolytes well, the precipitate is gelatinous and difficult to process. The dried complexes, for example, are generally infusible and therefore cannot be injection molded or reformed into articles under elevated temperatures. See Michaels, A. S., J. Industrial Engin. Chem. 57, 32-40 (1965).
PCT application WO/2008/027989 describes methods of compacting polyelectrolyte complexes to form articles in the presence of a salt. The moduli of these articles, while significantly greater than the uncompacted materials, are significantly lower than expected. For example, the modulus of a complex of poly(diallyldimethylammonium) and poly(styrene sulfonate) (PDADMA/PSS) in the multilayer form was a maximum of about 17 MPa whereas the identical polyelectrolytes after compaction by the method described in PCT application WO/2008/027989 was only about 1 MPa. Although many applications, especially those related to the damping of mechanical vibrations, require materials with moduli in the kPa-MPa range, other applications benefit from materials of higher strength, those having moduli in the MPa and above range. There is a need to transform solution precipitated polyelectrolyte complexes into the highest-strength material possible.
Polyelectrolyte complexes have been proposed as tissue engineering scaffolding (e.g., see Lim and Sun, Science, 210:908-910 (1980) and Yu et al., U.S. Pat. No. 6,905,875). The purpose of a tissue engineering scaffold is to support and maintain growing cells. Thus, these scaffolds are usually soft and porous and, therefore, not well suited for use as a compressive mechanical support. A tissue engineering scaffold is typically designed, prepared and employed without designing or expecting a particular damping property.